Somatotropin is a protein-based peptide hormone that is secreted by the pituitary gland and is responsible for the growth of young animals into their adult size. In humans, it mediates the growth of children to their adult height and stimulates the cells in many tissues. Also called growth hormone (GH), it has also been used by athletes since at least the 1970s to increase muscle mass and injury recovery speeds and has been promoted as an anti-aging drug.

GH causes genes in the nucleus of its target cells to increase production of proteins that are necessary for cell growth. All hormones operate by binding to a receptor on the surface of their target cells, then changing the activity of genes in that cell's nucleus. Most cells in the body will respond to somatotropin in circulation, but there are specific targets that it regulates to induce growth. Chief among these are the liver cells that secrete insulin-like growth factor 1 (IGF-1). This compound is responsible for the effects of somatotropin that characterize children's growth, such as height increases.

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When IGF-1 binds to cartilage cells, these differentiate to make new bone. The higher the concentration of these hormones, the more the bone mass increases. Thus the highest levels of both growth hormone and IGF-1 in the human body are reached during the puberty growth spurt, when much new bone is being made. Over the course of a lifetime, GH concentration gradually diminishes, and healthy adults will have levels less than two-thirds of those found in a young adolescent. Growth hormone has many effects on the normal adult metabolism, and they include promoting the metabolism of fats, new protein synthesis and the production of glucose in the liver.

Somatotropin is secreted by the anterior pituitary gland, where it is made by the cells called somatotrophs. These and all other pituitary cells respond to the release of precursor hormones by the hypothalamus by increasing or decreasing the amounts of the hormone they produce. The hypothalamus sends precursor or “releasing hormones” to the pituitary by secreting them into its blood vessels. The precursor to growth hormone is called somatotropin-releasing hormone (SRH). The hypothalamus secretes SRH whenever the body needs more growth hormone in circulation.

Various physiological states alert the hypothalamus to the need for more growth hormone production, such as age, nutrition, gender, physical activity and hormonal levels. Even the time of day is important, because more growth hormone is secreted during sleep than during waking hours. Somatotropin production is slowed or stopped when the hypothalamus releases the hormone somatostatin; an increase in the concentration of GH in the bloodstream leads to release of somatostatin by the hypothalamus, which in turn causes the anterior pituitary to stop secreting GH.

A synthetic version of somatotropin is called human growth hormone (HGH), or somatropin. It is a recombinant protein manufactured using molecular biology technology; previously, growth hormone was obtained from the pituitary glands of cadavers. Somatropin is used by some athletes as an anabolic steroid to improve athletic performance, to help muscles recover more quickly from injury and to increase the ratio of lean muscle mass to fat. There is no scientific consensus that HGH increases strength, just that it promotes muscle growth.

HGH is used clinically to treat children who are not producing enough GH in their own bodies for normal growth. Children who have somatotropin deficiencies typically have short statures and often appear younger than they are. The disease can also retard the onset of puberty. HGH treatment reverses this condition.

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